Curable silicone coated gasket and composition

ABSTRACT

A gasket of a nonwoven sheet material, and a coating composition on at least one side of the nonwoven sheet material. The coating composition is a reaction product of a vinyl functional polydimethylsiloxane, a trimethylsiloxy silica terpolymer, a reactive diluent, and a catalyst. The reactive diluent can be a methyl vinyl cyclic siloxane having the formula (MeViSiO) n  wherein n is an integer of from three to five, Me is methyl, and Vi is vinyl.

BACKGROUND OF THE INVENTION

This invention relates to a gasket material, and more particularly, toan automotive gasket of a paper base material bearing a coating of asilicone material.

A gasket is a deformable layer of packing material that is designed tobe firmly held between at least two contact surfaces having a jointintended to be sealed with the gasket in order to prevent leakage fromthe joint. The basic function of a gasket is to fill the space betweentwo imperfect faces sought to be joined in a fluid tight relationship.The use of a gasket eliminates the need for costly finishing of thefaces which would otherwise be necessitated. Gaskets are manufactured inmany varied shapes and sizes but in general are flat thin materialswhich can be compressed. They can be constructed of matural or syntheticrubber, felt, cork, vegetable fiber, paper, asbestos, plastic, and softmetallic materials such as lead and copper. The gasket may be employedin service in duties ranging from the prevention of leakage of liquids,to the rendering of certain joints to be gas tight such as hightemperature engine exhaust manifolds. The ideal gasket is a resilientpredictably compressible composition of one or more materials whichconforms easily to joint surface irregularities, and which cancompensate for joint distortion during operation and thermalfluctuations.

For most applications, the gasket material alone is not sufficient toform a seal due to the many voids and channels that exist in thematerial. It has therefore become common practice to incorporateadditives and impregnants into the gasket material such as fillers,binders, sealants, and saturants, of varied and complex description, inan attempt to compensate for these voids and channels, thereby renderingthe gasket material to be more leak proof.

For example, representative of additives and impregnants which have beenused in the past are glue, grease, wax, rubber such asstyrene-butadiene, plastics, phenolic resins, elastomers such aschloroprene, nitriles, and fluoroelastomers, and silicones. Thus, inU.S. Pat. No. 2,956,908, issued Oct. 18, 1960, there is described agasket matrial specific to automotive vehicle application and whereinthere is included as a coating thereon a mixture of a silicone fluid andcastor oil. A mixture of chrysotile asbestos and a heat curable siloxaneelastomer is taught in U.S. Pat. No. 3,453,228, issued July 1, 1969,although the use of such material as a gasket is not specificallydisclosed. However, a cylinder head gasket including a silicone rubberbinder is shown in U.S. Pat. No. 4,355,068, issued Oct. 19, 1981.

Thus, it should be apparent that the use of silicone materials ingaskets is not new. What is new, however, and covered by the presentinvention, is a coating for a gasket material having improved waterresistance.

SUMMARY OF THE INVENTION

This invention relates to a gasket of a nonwoven sheet material, and acoating composition on at least one side of the nonwoven sheet material.The coating composition is a reaction product of a vinyl functionalpolydimethylsiloxane, a trimethylsiloxy silica terpolymer, and areactive diluent.

The invention also relates to a method of rendering a gasket leakresistant in which a gasket is formed from a nonwoven sheet material.There is applied to at least one side of the nonwoven sheet material acoating composition, and the coating composition is the reaction productof a reactive diluent and a curable silicone composition which comprisesa resinous polymeric siloxane containing silicon-bonded hydroxylradicals and consisting essentially of R₃ SiO_(1/2) siloxane units andSiO_(4/2) siloxane units wherein the ratio of the number of said R₃SiO_(1/2) siloxane units to the number of said SiO_(4/2) siloxane unitshas a value of from 0.6/1 to 0.9/1 and each R denotes, independently, amonovalent hydrocarbon radical, an organohydrogenpolysiloxane whereineach organic radical is, independently, a monovalent hydrocarbonradical, there being an average of at least one silicon-bonded hydrogenradical per molecule of said organohydrogenpolysiloxane, and apolydiorganosiloxane having the formula

    ZR'".sub.2 SiO(R'".sub.2 SiO).sub.c SiR'".sub.2 Z

wherein Z denotes hydroxyl or R'" and each R'" denotes independently, amonovalent hydrocarbon radical, there being an average of at least twovinyl radicals or silicon-bonded hydroxyl radicals per molecule and chas a value sufficient to provide a viscosity of from 0.1 to 10,000 Pa.sat twenty-five degrees Centigrade for the polydiorganosiloxane.

In a preferred embodiment of the present invention, the reactive diluentis a vinyl cyclic siloxane having the formula (QViSiO)_(n) wherein n isan integer of from three to five, Q is methyl, phenyl, or hexenyl, andVi is vinyl. The reactive diluent may include combinations of vinylcyclic siloxane trimer, tetramer, and pentamer. The ratio of the vinylcomponent of the coating composition to silica can be 1:3, and thecoating composition has a viscosity less than about one thousandcentipoise measured at twenty-five degrees Centigrade; or the coatingcomposition can have a viscosity less than about four hundred centipoisemeasured at twenty-five degrees Centigrade; or the ratio of the vinylcomponent of the coating composition to silica can be 3:1, and with theviscosity of the coating composition being in excess of about fivethousand centipoise measured at twenty-five degrees Centigrade.

In some additional more specific embodiments of the present invention,there is provided the step of coating one or both sides of the nonwovensheet material with the coating composition and the step of curing thecoated nonwoven sheet material. The coated and cured nonwoven sheetmaterial may then be heat aged for a predetermined period of time.Preferably, the coated, cured, nonwoven sheet material contains fromabout one to about fifteen percent by weight of the coating compositionbased on the total weight of the gasket nonwoven sheet material.

It is therefore the object of the present invention to provide anonwoven sheet material that can be used as a gasket for automotivevehicle applications, and in which the gasket has been so treated inorder to render the material more resistant to fluid leakage than hasbeen heretofore possible with prior art treatment techniques.

These and other features, objects, and advantages, of the hereindescribed present invention will become more apparent when considered inconjunction with the following detailed description thereof.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, there is provided a silicone saturantautomotive gasket coating. A nonwoven base paper is employed to form thegasket, and with the silicone composition of the present inventioncoated thereon, and preferably cured. The thusly coated gasket possessesimproved water resistance. Saturation of the nonwoven base paper withthe heat curable silicone composition of the present invention alsoresults in improved resistance of the gasket to oils, fuels, andwater-glycol materials including typical automotive antifreezeformulations.

It has been found that prior art gasket coating materials do notpenetrate sufficiently into the base material sheet of the finishedgasket in order to completely fill the voids and channels that exist inthe base sheet. Such prior art materials further have not provided acomplete seal but allow wicking and bleeding of many materials such asantifreeze formulations, which result in migration of fluids from thejoint sought to be sealed because of the defective coating employedduring the manufacture of the gasket. This leakage of fluids not onlytends to deplete necessary vehicle fluids, but the leaking fluids form afilm on vehicle components which attract dirt and debris.

The compositions of the present invention, however, have been found topenetrate the base material of the gasket to a greater degree thanheretofore known in the prior art. This is due to the fact that informulating the compositions used herein, a reactive diluent provides aproduct of lower viscosity and hence of better penetration capabilities.The compositions of the present invention exclude fluids to a greaterdegree both in the uncured and cured states of the compositions, andwith a lower viscosity penetrate the base material and fill the variousvoids and channels. The base material it has been found, should includefrom one to fifteen, preferably from three to six percent by weight ofthe composition of the present invention based on the total weight ofthe base material.

Any type of gasket base sheet material may be employed in accordancewith the concept of the present invention. However, the preferred gasketbase sheet material is a nonwoven gasket paper which constitutes aboutninety-two weight percent of a fiber mat, and about eight weight percentof a nitrile rubber binder. The fiber mat may include polyester fibers,ceramic fibers, polyaramids, fiberglass, cellulosic fibers, or mixturesand combinations thereof. The gasket base sheet material should besaturated with the coating of the present invention in order to insureagainst leakage and weeping of fluids through the joint being sealed.While described herein as being specific to automobile type usages, thegaskets prepared in accordance with the present invention can be used inany application where a fluid type seal is required such as inequipment, vehicles, aircraft, or machinery.

The composition of the present invention which is used to coat thegasket base sheet is the reaction product of a vinyl functionalpolydimethylsiloxane, a trimethylsiloxy silica terpolymer, and areactive diluent. The vinyl functional polydimethylsiloxane and thetrimethylsiloxy silica terpolymer are disclosed in U.S. Pat. No.4,322,518, issued Mar. 30, 1982, the disclosure of which is incorporatedherein by reference. This curable silicone composition can be bestdescribed as (i) a resinous polymeric siloxane containing silicon-bondedhydroxyl radicals and consisting essentially of R₃ SiO_(1/2) siloxaneunits and SiO_(4/2) siloxane units wherein the ratio of the number ofsaid R₃ SiO_(1/2) siloxane units to the number of said SiO_(4/2)siloxane units has a value of from 0.6/1 to 0.9/1 and each R denotes,independently, a monovalent hydrocarbon radical, (ii) ororganohydrogenpolysiloxane wherein each organic radical is,independently, a monovalent hydrocarbon radical, there being an averageof at least one silicon-bonded hydrogen radical per molecule of saidorganohydrogenpolysiloxane, and (iii) a polydiorganosiloxane having theformula

    ZR'".sub.2 SiO(R'".sub.2 SiO).sub.c SiR'".sub.2 Z

wherein Z denotes hydroxyl or R'" and each R'" denotes independently, amonovalent hydrocarbon radical, there being an average of at least twovinyl radicals or silicon-bonded hydroxyl radicals per molecule and chas a value sufficient to provide a viscosity of from 0.1 to 10,000 Pa.sat twenty-five degrees Centigrade for the polydiorganosiloxane.

The above described curable silicone composition is reacted with areactive diluent in order to produce the coating compositions of thepresent invention. The reactive diluent of preference is a methyl vinylcyclic siloxane having the formula (MeViSiO)_(n) wherein n is an integerof from three to five, Me is methyl, and Vi is vinyl. The reactivediluent can include combinations of methyl vinyl cyclic siloxane trimer,tetramer, and pentamer. The reaction product of the curable siliconecomposition and the reactive diluent may be tailored to a variety offormulations including, for example, a formulation hereinafteridentified as "Formulation A" wherein the ratio of vinyl polymer tosilica is 1:3 and having a viscosity of 940 centipoise measured attwenty-five degrees Centigrade. "Formulation B" of the reaction productof the curable silicone composition and the reactive diluent includesvinyl cyclics but without the vinyl polymer component and has aviscosity of 360 centipoise measured at twenty-five degrees Centigrade.In "Formulation C", the ratio of the vinyl polymer component to thesilica is the reverse of Formulation A or 3:1, and with the viscosity ofthe Formulation C material being 5,160 centipoise measured attwenty-five degrees Centigrade. Obviously, numerous other formulationsand combinations are possible within the scope of the present invention.

The following example illustrates the use of the compositions of thepresent invention.

EXAMPLE I

A compressed automotive gasket material and a highly compressedautomotive gasket material were selected for testing. The compressedmaterial is referred to as "soft" in the following Tables whereas thehighly compressed gasket material is referred to in the Tables as"hard". Both the soft and hard materials were standard nonwoven gasketpaper sheet stock material containing a binder. Formulations A, B, andC, as noted above, were each used to treat each type of sheet stock. AControl sheet stock material containing no coating of the composition ofthe present invention was employed in each instance. Each ofFormulations A-C was drawn down on a gasket sheet stock using a smoothsteel Meyer rod. Some samples of the sheet stock of gasket material werecoated on one side, while others were coated on both sides of the sheetstock, using each Formulations A-C. Each coated gasket was treatedfollowing the coating step, and before the coating was cured, for weightadded; water absorption at the end of twenty-four hours at temperaturesof twenty-three and sixty degrees Centigrade; tensile strength; andpercent elongation. These results are shown in Tables I-II. Each coatedgasket was placed in an oven for five minutes and at a temperature ofone hundred-fifty degrees Centigrade in order to facilitate the cure thecoating in each instance. The cured gaskets were each again evaluated aspreviously, and in addition, were each further evaluated for the amountof weight loss occurring during the curing process. These results areshown in Tables III-VI. The coated gaskets in Tables III-VI additionallywere soaked in the appropriate Formulation for a period of ten minutes.Tables VII and VIII show the results of heat aging of the previouslytreated and cured gaskets. These coated and cured gaskets weremaintained at a temperature of one hundred-fifty degrees Centigrade forseven days and evaluated as previously indicated. This heat aging stepwas intended to assimilate actual conditions encountered duringoperation of a typical automobile engine.

                                      TABLE I                                     __________________________________________________________________________    COATED HARD GASKET: ONE SIDE                                                  UNCURED                                                                                                  Tensile                                                         % Water Absorption                                                                          Strength                                           Formulation                                                                          % Add-On                                                                            24 Hrs/R.T.                                                                          24 Hrs/60° C.                                                                 PSI  Elongation %                                  __________________________________________________________________________    A      8.7   17.3   16.3   695  1.33                                          B      5.7   16.9   17.2   467  0.67                                          C      7.7   13.7   14.1   252  5.0                                           D      3.9   14.9   15.4   661  1.67                                          Control      15.4   16.2   659  9.0                                           __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    COATED SOFT GASKET: ONE SIDE                                                  UNCURED                                                                                                  Tensile                                                         % Water Absorption                                                                          Strength                                           Formulation                                                                          % Add-On                                                                            24 Hrs/R.T.                                                                          24 Hrs/60° C.                                                                 PSI  Elongation %                                  __________________________________________________________________________    A      9.7   34.0   38.0   610  2.0                                           B      8.0   43.9   45.3   718  2.33                                          C      6.7   41.1   44.6   739  2.0                                           D      4.6   49.6   47.9   806  2.0                                           Control      71.0   75.5   786  2.0                                           __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________    COATED HARD GASKET: ONE SIDE                                                  CURED 5 MINUTES @ 150° C.                                                                               Tensile                                                         % Water Absorption                                                                          Strength                                     Formulation                                                                          % Add-On                                                                            % Wt Loss                                                                           24 Hrs/R.T.                                                                          24 Hrs/60° C.                                                                 PSI  Elongation %                            __________________________________________________________________________    A      8.7   2.3   18.3   15.7   754  0.33                                    B      5.7   1.8   18.7   15.9   434  0.33                                    C      7.7   1.8   12.4   13.0   460  3.0                                     D      3.9   1.9   13.8   15.3   319  1.0                                     Control      0.85  17.7   10.9   610  6.0                                     __________________________________________________________________________

                  TABLE IV                                                        ______________________________________                                        COATED HARD GASKET: BOTH SIDES                                                CURED 5 MINUTES @ 150° C.                                                     %      %        % Water Absorption                                     Formulation                                                                            Add-On   Wt Loss  24 Hrs/R.T.                                                                            24 Hrs/60° C.                      ______________________________________                                        A        30.5     3.5      11.4     10.7                                      B        31.3     3.4      11.5     9.1                                       C        17.3     3.7      10.8     11.7                                      D        43.2     11.6     9.4      8.9                                       Control           0.85     17.7     10.9                                      ______________________________________                                    

                                      TABLE V                                     __________________________________________________________________________    COATED SOFT GASKET: ONE SIDE                                                  CURED 5 MINUTES @ 150° C.                                                                               Tensile                                                         % Water Absorption                                                                          Strength                                     Formulation                                                                          % Add-On                                                                            % Wt Loss                                                                           24 Hrs/R.T.                                                                          24 Hrs/60° C.                                                                 PSI  Elongation %                            __________________________________________________________________________    A      9.7   2.3   34.6   34.5   795  1.0                                     B      8.0   1.9   36.6   42.3   828  0.33                                    C      6.7   2.3   31.2   37.6   764  4.0                                     D      4.6   2.1   22.8   29.1   845  2.0                                     Control      1.2   31.5   42.1   786  1.0                                     __________________________________________________________________________

                  TABLE VI                                                        ______________________________________                                        COATED SOFT GASKET: BOTH SIDES                                                CURED 5 MINUTES @ 150° C.                                                     %      %        % Water Absorption                                     Formulation                                                                            Add-On   Wt Loss  24 Hrs/R.T.                                                                            24 Hrs/60° C.                      ______________________________________                                        A        40.5     4.5      16.7     14.7                                      B        44.5     1.9      7.9      11.8                                      C        16.9     4.5      13.1     17.3                                      D        59.0     14.2     13.7     11.8                                      Control           1.2      31.5     42.1                                      ______________________________________                                    

                                      TABLE VII                                   __________________________________________________________________________    COATED HARD GASKET: ONE SIDE                                                  CURED 5 MINUTES @ 150° C.                                              HEATED AGE 7 DAYS @ 150° C.                                                                             Tensile                                                         % Water Absorption                                                                          Strength                                     Formulation                                                                          % Add-On                                                                            % Wt Loss                                                                           24 Hrs/R.T.                                                                          24 Hrs/60° C.                                                                 PSI  Elongation %                            __________________________________________________________________________    A      8.7   1.4   8.5    10.0   95   0.0                                     B      5.7   0.0   8.8    11.4   126  0.33                                    C      7.7   0.8   9.3    10.7   132  0.67                                    D      3.9   0.0   8.6    11.7   184  0.0                                     Control      1.0   20.6   71.9   44   0.33                                    __________________________________________________________________________

                                      TABLE VIII                                  __________________________________________________________________________    COATED SOFT GASKET: ONE SIDE                                                  CURED 5 MINUTES @ 150° C.                                              HEAT AGED 7 DAYS @ 150° C.                                                                                Tensile                                                         % Water Absorption***                                                                       Strength                                   Formulation                                                                          % Add-On*                                                                            % Wt. Loss**                                                                         24 Hrs/R.T.                                                                          24 Hrs/60° C.                                                                 PSI  Elongation %                          __________________________________________________________________________    A      9.7    0.6    11.8   21.4   143  0.67                                  B      8.0    0.3    12.5   22.3   185  0.33                                  C      6.7    0.0    12.2   22.6   167  0.33                                  D      4.6    0.0    11.4   20.0   122  0.33                                  Control       1.25   16.3   73.4   710  0.0                                   __________________________________________________________________________     ##STR1##                                                                      -                                                                             ##STR2##                                                                      -                                                                             ##STR3##                                                                      -                                                                             R.T. = Room Temperature or 23° C.                                 

It can be seen from Tables I-VIII that the coated gaskets in theuncured, cured and thermally aged stages improve the water resistance ofthe gasket material. This is particularly apparent from a study ofTables II and Tables VI-VII. Such improvement in water exclusionrendered by the compositions of the present invention occurs regardlessof whether the coating is applied to only one side of the gasket or toboth sides thereof. The Tables also clearly indicate that only slightamounts of material is lost during cure, as well as during the heataging process.

There was also included in the foregoing tests, in addition to theControl, a Formulation "D" which is a composition of an equilibriatedblend of dimethyl cyclic siloxanes of the formula (Me₂ SiO)_(n) whereinMe is methyl and n is an integer from three to five, and an alkoxysilane such as methyl trimethoxy silane or phenyl trimethoxy silane,catalyzed with a tetrabutyl titanate transition-type catalyst.Formulation D has a viscosity of the order of about sixty centipoisemeasured at twenty-five degrees Centigrade.

The following Examples illustrate procedures for making each ofFormulations A-D referred to above.

EXAMPLE II (Formulation A)

A mixture of (i) 41.2 parts of an organosilicon resin composed of Me₃SiO_(1/2) units and SiO_(4/2) units in the ratio of 0.6 Me₃ SiO_(1/2)units to 1 SiO_(4/2) unit: (ii) 41.2 parts of a polymer having theaverage formula Me₃ SiO(MeHSiO)₃₅ SiMe₃ ; and 17.6 parts of xylene wascharged to a vessel provided with a stirrer.

This mixture, with continuous stirring, was heated to a temperature of150 degrees Centigrade. The pressure in the vessel was gradually reducedto a value between 40 mm and 50 mm Hg. Heating under reduced pressurewas continued for approximately 2 hours. During this time, volatilizedxylene was removed and condensed in a reservoir separate from the vesselcontaining the mixture. After 2 hours, the reaction product wasrecovered. It was substantially solvent free liquid organosilicon resin.

A composition of this invention was prepared by mixing 300 parts of theabove-prepared organosilicon resin, 100 parts of a vinyl-terminatedpolydimethylsiloxane having a viscosity of approximately 35,000centipoise and 750 dimethylsiloxy units per molecule; 1.48 parts of aninhibitor such as 3,5-dimethyl-1-hexyn-3-ol, 0.5 parts of a catalystprepared by reacting chloroplantinic acid withdivinyltetramethyldisiloxane according to the method of U.S. Pat. No.3,419,593, 38.9 parts of a silicone resin composed of Me₂ ViSiO_(1/2)units, Me₃ SiO_(1/2) units and SiO_(4/2) units in the ratio of0.15:0.6:1, and 155.6 parts of a mixture ofpolymethylvinylcyclosiloxanes having from 4 to 10 silicon atoms permolecule. The function of the inhibitor is to prevent the reaction ofthe catalyst with the silicone materials prior to removal of theinhibitor.

EXAMPLE III (Formulation C)

A composition of this invention was prepared by mixing 33.3 parts of theabove-prepared organosilicon resin, 100 parts of the vinyl-terminatedpolydimethylsiloxane described in Example II, 0.167 parts of3,5-dimethyl-1-hexyn-3-ol, 0.5 parts of the catalyst described inExample II, 38.9 parts of the silicone resin composed of Me₂ ViSiO_(1/2)units, Me₃ SiO_(1/2) units and SiO_(4/2) units described in Example II,and 51.85 parts of polymethylvinylcyclosiloxanes having from 4 to 10silicon atoms per molecule.

EXAMPLE IV (Formulation B)

A composition of this invention was prepared by mixing 100 parts of theabove-prepared organosilicon resin, 0.5 parts of3,5-dimethyl-1-hexyn-3-ol, 0.13 parts of the catalyst described inExample II, and 33.3 parts of polymethylvinylcyclosiloxanes having from4 to 10 silicon atoms per molecule.

EXAMPLE V (Formulation D)

A composition of this invention was prepared by mixing 100 parts ofmethoxy terminated, polydimethylsiloxane polymers with methylsilsesquioxanes containing a monomethyl to dimethyl polysiloxane rationof 3:1 and containing 21.1 weight percent methoxy with 3 parts oftetrabutyltitanate catalyst.

It will be apparent from the foregoing that many other variations andmodifications may be made in the structures, compounds, compositions,articles of manufacture, and methods described herein without departingsubstantially from the essential features and concepts of the presentinvention. Accordingly, it should be clearly understood that the formsof the invention described herein are exemplary only and are notintended as limitations on the scope of the present invention.

That which is claimed is:
 1. A gasket comprising a nonwoven sheetmaterial, and a coating composition on at least one side of the nonwovenwheet material, the coating composition being a reaction product of avinyl functional polydimethylsiloxane, a trimethylsiloxy silicaterpolymer, and a reactive diluent, the reactive diluent being a vinylcyclic siloxane having the formula (QViSiO)_(n) wherein n is an integerof from three to five, Q is methyl, phenyl, or hexenyl, and Vi is vinyl.2. A gasket according to claim 1 wherein the reactive diluent includescombinations of vinyl cyclic siloxane timer, tetramer, and pentamer. 3.A gasket according to claim 2 wherein the ratio of the vinyl componentof the coating composition to silica is 1:3, and the coating compositionhas a viscosity less than about one thousand centipoise measured attwenty-five degrees Centigrade.
 4. A gasket according to claim 2 whereinthe coating composition has a viscosity less than about four hundredcentipoise measured at twenty-five degrees Centigrade.
 5. A gasketaccording to claim 2 wherein the ratio of the vinyl component of thecoating composition to silica is 3:1, and the viscosity of the coatingcomposition is in excess of about five thousand centipoise measured attwenty-five degrees Centigrade.
 6. A gasket according to claim 1 whereinthe reaction product includes (i) a resinous polymeric siloxanecontaining silicon-bonded hydroxyl radicals and consisting essentiallyof R₃ SiO_(1/2) siloxane units and SiO_(4/2) siloxane units wherein theratio of the number of said R₃ SiO_(1/2) siloxane units to the number ofsaid SiO_(4/2) siloxane units has a value of from 0.6/1 to 0.9/1 andeach R denotes, independently, a monovalent hydrocarbon radical, (ii) anorganohydrogenpolysiloxane wherein each organic radical is,independently, a monovalent hydrocarbon radical, there being an averageof at least one silicon-bonded hydrogen radical per molecule of saidorganohydrogenpolysiloxane, and (iii) a polydiorganosiloxane having theformula

    ZR'".sub.2 SiO(R'".sub.2 SiO).sub.c SiR'".sub.2 Z

wherein Z denotes hydroxyl or R'" and each R'" denotes independently, amonovalent hydrocarbon radical, there being an average of at least twovinyl radicals or silicon-bonded hydroxyl radicals per molecule and chas a value sufficient to provide a viscosity of from 0.1 to 10,000 Pa.sat twenty-five degrees Centigrade for the polydiorganosiloxane.
 7. Agasket according to claim 6 wherein both sides of the nonwoven sheetmaterial are coated with the coating composition.
 8. A gasket accordingto claim 7 wherein the coated nonwoven sheet material is cured.
 9. Agasket according to claim 8 wherein the reaction product includes acatalyst and an inhibitor.
 10. A gasket according to claim 9 wherein thecoated, cured, nonwoven sheet material contains from about one to aboutfifteen percent by weight of the coating composition based on the totalweight of the gasket nonwoven sheet material.
 11. A coating compositionfor nonwoven sheet materials comprising a reaction product of (i) areactive diluent, (ii) a resinous polymeric siloxane containingsilicon-bonded hydroxyl radicals and consisting essentially of R₃SiO_(1/2) siloxane units and SiO_(4/2) siloxane units wherein the ratioof the number of said R₃ SiO_(1/2) siloxane units to the number of saidSiO_(4/2) siloxane units has a value of from 0.6/1 to 0.9/1 and each Rdenotes, independently, a monovalent hydrocarbon radical, (iii) anorganohydrogenpolysiloxane wherein each organic radical is,independently, a monovalent hydrocarbon radical, there being an averageof at least one silicon-bonded hydrogen radical per molecule of saidorganohydrogenpolysiloxane, and (iv) a polydiorganosiloxane having theformula

    ZR'".sub.2 SiO(R'".sub.2 SiO).sub.c SiR'".sub.2 Z

wherein Z denotes hydroxyl or R'" and each R'" denotes independently, amonovalent hydrocarbon radical, there being an average of at least twovinyl radicals or silicon-bonded hydroxyl radicals per molecule and chas a value sufficient to provide a viscosity of from 0.1 to 10,000 Pa.sat twenty-five degrees Centigrade for the polydiorganosiloxane, thereactive diluent being a methyl vinyl cyclic siloxane having the formula(MeViSiO)_(n) wherein n is an integer of from three to five, Me ismethyl, and Vi is vinyl.
 12. The coating composition of claim 11 whereinthe reactive diluent includes combinations of cyclic siloxane trimer,tetramer, and pentamer.
 13. The coating composition of claim 12 whereinthe ratio of the vinyl component of the reaction product to silica is1:3, and the reaction product has a viscosity less than about onethousand centipoise measured at twenty-five degrees Centigrade.
 14. Thecoating composition of claim 12 wherein the reaction product has aviscosity less than about four hundred centipoise measured attwenty-five degrees Centigrade.
 15. The coating composition of claim 12wherein the ratio of the vinyl component of the reaction product tosilica is 3:1, and the viscosity of the reaction product is in excess ofabout five thousand centipoise measured at twenty-five degreesCentigrade.
 16. A method of rendering a gasket leak resistant comprisinga forming a gasket from a nonwoven sheet material, applying to at leastone side of the nonwoven sheet material a coating composition, thecoating composition being a reaction product of a vinyl functionalpolydimethylsiloxane, a trimethylsiloxy silica terpolymer, and areactive diluent, the reactive diluent being a methyl vinyl cyclicsiloxane having the formula (MeViSiO)_(n) wherein n is an integer offrom three to five, Me is methyl, and Vi is vinyl.
 17. A methodaccording to claim 16 wherein the reaction product includes (i) aresinous polymeric siloxane containing silicon-bonded hydroxyl radicalsand consisting essentially of R₃ SiO_(1/2) siloxane units and SiO_(4/2)siloxane units wherein the ratio of the number of said R₃ SiO_(1/2)siloxane units to the number of said SiO_(4/2) siloxane units has avalue of from 0.6/1 to 0.9/1 and each R denotes, independently, amonovalent hydrocarbon radical, (ii) an organohydrogenpolysiloxanewherein each organic radical is, independently, a monovalent hydrocarbonradical, there being an average of at least one silicon-bonded hydrogenradical per molecule of said organohydrogenpolysiloxane, and (iii) apolydiorganosiloxane having the formula

    ZR'".sub.2 SiO(R'".sub.2 SiO).sub.c SiR'".sub.2 Z

wherein Z denotes hydroxyl or R'" and each R'" denotes independently, amonovalent hydrocarbon radical, there being an average of at least twovinyl radicals or silicon-bonded hydroxyl radicals per molecule and chas a value sufficient to provide a viscosity of from 0.1 to 10,000 Pa.sat twenty-five degrees Centigrade for the polydiorganosiloxane.
 18. Amethod according to claim 16 wherein the reactive diluent includescombinations of cyclic siloxane trimer, tetramer, and pentamer.
 19. Amethod according to claim 18 wherein the ratio of the vinyl component ofthe coating composition to silica is 1:3, and the coating compositionhas a viscosity less than about one thousand centipoise measured attwenty-five degrees Centigrade.
 20. A method according to claim 18wherein the coating composition has a viscosity less than about fourhundred centipoise measured at twenty-five degrees Centigrade.
 21. Amethod according to claim 18 wherein the ratio of the vinyl component ofthe coating composition to silica is 3:1, and the viscosity of thecoating composition is in excess of about five thousand centipoisemeasured at twenty-five degrees Centigrade.
 22. A method according toclaim 18 including the step of coating both sides of the nonwoven sheetmaterial with the coating composition.
 23. A method according to claim22 including the step of curing the coated nonwoven sheet material. 24.A method according to claim 23 wherein the reaction product includes acatalyst and an inhibitor.
 25. A method according to claim 24 whereinthe coated, cured, nonwoven sheet material contains from about one toabout fifteen percent by weight of the coating composition based on thetotal weight of the gasket nonwoven sheet material.
 26. A gasketcomprising a nonwoven sheet material, and a coating composition on atleast one side of the nonwoven sheet material, the coating compositionbeing a mixture of a methyl terminated polydimethylsiloxane polymer, amethyl silsesquioxane containing a monomethyl to dimethyl polysiloxaneratio of 3:1, and a catalyst.
 27. A gasket according to claim 26 whereinboth sides of the nonwoven sheet material are coated with the coatingcomposition.
 28. A gasket according to claim 27 wherein the coatednonwoven sheet material is cured.
 29. A gasket according to claim 28wherein the coated, cured, nonwoven sheet material contains from aboutone to about fifteen percent by weight of the coating composition basedon the total weight of the gasket nonwoven sheet material.
 30. A methodof rendering a gasket leak resistant comprising forming a gasket from anonwoven sheet material, applying to at least one side of the nonwovensheet material a coating composition, the coating composition being amixture of a methoxy terminated polydimethylsiloxane polymer, a methylsilsesquioxane containing a monomethyl to dimethyl polysiloxane ratio of3:1, and a catalyst.
 31. A method according to claim 30 including thestep of coating both sides of the nonwoven sheet material with thecoating composition.
 32. A method according to claim 31 including thestep of curing the coated nonwoven sheet material.
 33. A methodaccording to claim 32 wherein the coated, cured, nonwoven sheet materialcontains from about one to about fifteen percent by weight of thecoating composition based on the total weight of the gasket nonwovensheet material.